RomanCart and PHPbb

I have a situation I have a site i look after that has got a cart from a company called RomanCart all straight forward. Previous to this they just had Amember and PHPbb running. In Amember when they brought and item it automatically upgraded the user group in PHPbb to VIP. The problem i now have is they are now using RomanCart as there shop but still require once an item is purchased for the user to have the phpbb group upgraded to VIP.

The Key info:
The user has to login to PHPBB to access the shop.
The roman Cart does have a Active product Option. Once you purchase an item it invisibly redirects you to a url and back (I'm hoping someone can guide me here)

Is there something i can do such as create a page from amember that upgrades the user when accessed and then i could just direct the active product to this url and hey presto